Write heads are devices which convert an electrical signal into a magnetic force which magnetizes a localized area of a magnetic storage medium such as a surface of a magnetic disk. The surface of the disk is magnetized in different directions according to a pattern derived from a code to store data on the disk.
A diagram of a conventional write head 8 is shown in FIG. 1. A coiled inductor 10 is wrapped around a magnetic core 12 which is suspended above a magnetic disk 14. The core 12 includes a gap 16. Current is drawn through the inductor 10 in a forward direction which induces a forward magnetic field in the core 12. The forward magnetic field traverses the gap 16 near a top surface 18 of the disk 14 and magnetizes the surface 18 in a forward direction. The direction of current in the inductor 10 may be reversed to generate a reverse magnetic field in the core 12. The surface 18 of the disk 14 near the gap 16 is then magnetized in a reverse direction. Periodic changes in the direction of the magnetism in the surface of the disk 14 may be created by moving the write head 8 relative to the disk 14 and changing the direction of current in the inductor 10 to store data according to well-known methods.
A conventional circuit 20 for controlling an inductor 22 for a write head is shown in FIG. 2. An H-bridge circuit 24 drives current into the inductor 22 according to a control signal generated by a control logic circuit 26. The H-bridge circuit 24 includes a first high-side transistor 28 and a first low-side transistor 30 connected in series between a voltage source V.sub.cc and a ground voltage reference. The H-bridge circuit 24 also includes a second high-side transistor 32 and a second low-side transistor 34 connected in series between the voltage source V.sub.cc and the ground voltage reference. The connection between the first high-side transistor 28 and the first low-side transistor 30 includes a first terminal 36 of the H-bridge circuit 24. Similarly, the connection between the second high-side transistor 32 and the second low-side transistor 34 includes a second terminal 38 of the H-bridge circuit 24.
The H-bridge circuit 24 drives current through the inductor 22 according to the control signal generated by the control logic circuit 26 and provided from a port 40. The control signal generated by the control logic circuit 26 is a digital control signal having a high voltage or a low voltage which controls a direction of current driven in the inductor 22. If the control signal is high, the high control signal is applied to render the second high side transistor 32 and the first low side transistor 30 conductive. The high control signal is inverted by an inverter 42 and applied to render the first high-side transistor 28 nonconductive. The high control signal is also inverted by an inverter 44 and is applied to the second low-side transistor 34 to render it nonconductive. Current is then directed from the voltage source V.sub.cc through the second high side transistor 32, the second terminal 38, the inductor 22, the first terminal 36, and then through the first low side transistor 30 to the ground voltage reference. As current flows through the inductor 22 from the second terminal 38 to the first terminal 36, a magnetic field is generated in a core wrapped by the inductor 22 in a first direction such that the core may magnetize a surface of a magnetic disk in the first direction.
Periodically the control logic circuit 26 reverses the voltage of the control signal provided at the port 40 to change the direction of the current in the inductor 22. If the control signal is switched from a high voltage to a low voltage, the second high side transistor 32 and the first low side transistor 30 are rendered nonconductive. The control signal is inverted by the inverters 42 and 44 such that the first high side transistor 28 and the second low side transistor 34 are rendered conductive. Current is then directed from the voltage source V.sub.cc through the first high side transistor 28, the first terminal 36, the inductor 22, the second terminal 38, and the second low side transistor 34 to the ground voltage reference. As current flows in the inductor 22 from the first terminal 36 to the second terminal 38 a magnetic field is generated in the core wrapped by the inductor 22 in a second direction. The core is now capable of magnetizing the surface of the magnetic disk in the second direction. The voltage of the control signal is switched rapidly when the circuit 20 is operating to change the direction of the magnetism in the surface of the disk such that data may be stored in the disk as described above.
When the direction of current in the inductor 22 is changed in response to a change in the control signal, oscillations occur in the current in the inductor 22 due to the existence of parasitic effects in the circuit 20. A plot of the current in the inductor 22 during a change in the control signal is shown in FIG. 3. The moment the control signal changes the current in the inductor 22 begins to change direction as shown by a point 50. Following the change in the control signal the current in the inductor 22 oscillates with an overshoot 52, an undershoot 54, and an overshoot 56 before settling to a steady current 58. The undershoot 54 is particularly hazardous because it can demagnetize the core wrapped by the inductor 22 and erase data stored nearby in a magnetic disk. The overshoot 52 may also, under some circumstances, put data stored on the magnetic disk at risk.
A conventional method for minimizing the oscillations shown in FIG. 3 is a modified circuit 60 shown in FIG. 4. The circuit 60 is similar to the circuit 20 shown in FIG. 2, and elements common to both circuits 60 and 20 have the same reference numerals.
The circuit 60 includes a damping resistor 62 connected between the first terminal 36 and the second terminal 38. The damping resistor 62 is thus coupled in parallel with the inductor 22. When the circuit 60 is operating to provide current to the inductor 22, some current is drawn through the damping resistor 62 bypassing the inductor 22. The damping resistor 62 damps oscillations in the current in the inductor 22 after the direction of the current has been changed. In particular, the damping resistor 62 reduces the number of oscillations and substantially minimizes any undershoot in the oscillations in the current in the inductor 22.
The reduction in the number of oscillations permits the current in the inductor 22 to settle to a steady amount more rapidly after a change in direction. After a steady current is reached an area of a magnetic disk is magnetized in a selected direction and the direction of the current in the inductor 22 may be reversed. The reduction in oscillations permits an increase in the frequency of directional changes in the current in the inductor 22 and therewith a reduction in the period of time needed to write data to a magnetic disk. In other words, one benefit of the damping resistor 62 is that the writing frequency of the write head may be increased.
While the damping resistor 62 moderates oscillations in current in the inductor 22, it also dissipates power as current is drawn through it. When the current in the inductor 22 is steady, there is a small voltage drop between the first terminal 36 and the second terminal 38 and power dissipation in the damping resistor 62 is minimal. However, when the direction of current in the inductor 22 is changed, a substantially higher voltage is applied across the inductor 22 and the damping resistor 62. As a result, the damping resistor 62 dissipates a substantial amount of power when the direction of the current in the inductor 22 is changed.